The folks over at GameIndutry chatted up EA's VP or corporate communications Jeff Brown recently about Activision defecting from the ESA and got the following quote:

"They're a big company and we feel that when you're a big company you've got a responsibility to consumers to work on policy issues which are very, very important to consumers. And the best way to do that is with an industry consensus. That's the way it's been done with the ESA in the US, and with ELSPA in the UK."

While it's not quite "With great power comes great responsibility" it's a pretty good statement about why Activision shouldn't have left the fold. He does take a few shots at the ESA leadership and having them prove their worth to the industry as they industry does need to maintain one solid/unified front. I'm still hopeful that the ESA can turn things around but we'll see if the ESA is open to change or not.